1. Field of the Invention
The present invention relates to building construction, and more specifically, to apparatus for anchoring shear walls to foundations and lower floors.
2. Background
Strong winds and earthquakes subject walls and others elements of a building to tremendous forces. If these forces are not distributed to the proper elements or structures capable of withstanding such force, the building may be torn apart. Foundations are often the strongest element of a building. Securely tying the walls of a building to the foundation greatly improves structural performance during periods of strong wind or earthquake. Securement promotes single body motion limiting whiplash amplification otherwise often resulting in structural failure.
Under extreme conditions, a building may be violently loaded or shaken back and forth in a lateral (side to side) direction. If a shear wall is tightly restrained at its base, loads may be smoothly transferred to the foundation. The loads may then be resolved in the foundation, where they appear as tension and compression forces.
Buildings are often composed of long walls, (walls with a length greater than the height) and short walls (walls that have a length shorter than the height). The tendency for a wall to lift vertically off a foundation is inversely proportional to the length of the wall. Tall narrow shear walls, which may be found in nearly all homes, act as lever arms and may magnify an imposed load. In certain instances, the actual load on the securement system may be magnified to several times the originally imposed load.
An as-built building is generally not the building that will be sustaining loads induced by wind or by earthquake shaking. Wood components of the building structure, including floors, joists, sill plates, top plates, and studs, will shrink. Shrinkage varies greatly but ranges typically from about one-quarter inch per floor (story) under the best of conditions, to well over one inch depending on the total cross-grain stack up (depth) of wood.
Wall securement may reduce or prevent lateral and vertical motion between the walls and the foundation. Additionally, it may be necessary to support the wall against forces that would tend to distort the wall's general rectangular shape. Building codes often require external and load bearing walls to be shear resistant by providing a plywood plane to support shear forces that may be imposed on the wall. Many times, building codes also require lateral and vertical securement of a wall to the foundation. Lateral and vertical securement may be accomplished by employing hold-downs, also referred to as tiedowns.
Hold-down systems are employed to secure walls of upper levels to walls of lower levels, as well as walls to foundations. Again the principle is to secure the entire structure to the foundation where structural forces can best be resolved. However, lower levels can present amplification of structural weaknesses to upper levels. If a hold-down system installed on a given level cannot compensate for all shrinkage and crushing affecting that level, structural weaknesses may be amplified on adjacent levels. Hold-down systems need to be able to compensate for structural weaknesses throughout the structure, and not just within a given level.
Moreover, hold-down systems can be difficult to install and expensive to fabricate. Some hold-down systems require assembly within narrow tolerances, making assembly difficult and time consuming. Other hold-down systems cannot compensate for structural weaknesses throughout the structure, causing an overload of a hold-down system on a given level. Some systems by major suppliers, when applied to multi-story buildings may accumulate slack with each floor. Some systems cannot hold down the highest floor of a building due to their operating principles and connection mechanisms Yet other hold-down approaches advance discretely between pre-determined, locking locations. Thus, at all points between the discreet locking points, such systems, leave a certain amount of slack, greatly compromising their effectiveness.
Hold-down systems are distributed about a building's floor plan and perimeter. They may also be distributed throughout the height of the building, in an attempt to secure each floor to the one below or to the foundation. Thus, the additive cost may be substantial for the numerous locations in a building warranting placement of hold-downs.
A need exists for a hold-down system that may be easily installed and utilizes the full potential of the system over the entire structure. It would be a further advancement to provide a hold-down system that may be produced and installed in greater quantities with greater speed and less expense. Likewise, it would be a further advance to develop a method of locating and installing hold-downs that would give the maximum rigidizing effect with a minimum number of hold-downs actually installed.